This is something you can change from your options menu, in the upper right click the button with three horizontal lines and select options. From there make sure you are under the General category on the left and then scroll down to the applications section. In this section you can chose the default action for opening file types, click on portable document format (.pdf) and if foxit is installed on the computer you should have it listed here. Once you change that foxit should pop up for you again.

that was my first step. Foxit Reader isn't listed as an option, that's why I wondered where it was. I have never installed it myself; it just always seemed to be there. Searches of Firefox show no mention of it anywhere.

Foxit isn't a firefox software and install problem you should direct that to foxit support. Also could be a 32 bit to 64 bit firefox install that could also remove previous unverified addons from running on the new install.

WestEnd said

Foxit isn't a firefox software

Correction, Foxit reader is not a Mozilla product nor is it built into Firefox.

Firefox is just the name of the web browser (since version 0.8) and not a company or such.

thank you, I didn't know that! I thought it was part of Firefox's capabilities. I'm not sure how I wound up with it originally, but I will head to Foxit and check it out now.

Hello oboe,

You won't be able to use the Foxit Reader plugin, as plugins are
no longer supported.

Maybe you'd like to give this add-on a try :

https://addons.mozilla.org/en-US/firefox/addon/open-in-foxit-reader/

Or not use Foxit, but instead : Options => General, under "Applications" => Portable Document Format (PDF) choose "Preview in Firefox"